1. Field of the Invention
The invention relates to methods of manufacturing alkaline batteries and more particularly to treatment of nickel plaques prior to impregnation and to the resultant product, and to cells containing the product.
2. Description of the Prior Art
A commercially established approach to the production of electrodes for alkaline batteries is to impregnate a porous supporting electrode structure (i.e., a porous nickel plaque) with finely divided active material so as to present a high surface area of a substantial amount of active material to the electrolyte. While loadings obtained in this way are quite satisfactory, higher loadings are desirable and reduced time required for loading is economically advantageous.
A number of impregnation procedures have been used in the past. Particularly simple was the procedure of soaking the porous plaque in a salt solution and evaporating the liquid. This was followed by soaking the plaque in a second solution to convert the soluble salt to an insoluble active form. Soaking in either the first or second solution, or both might be repeated several times to increase loading. Thermal decomposition is also used to convert to the insoluble active form. These procedures are referred to as chemical impregnation processes.
An alternative approach over these soaking processes is electrolytic impregnation (see L. Kandler U.S. Pat. No. 3,214,355 issued Oct. 26, 1965). In this process, active material is continuously deposited directly in the pores of the plaque. Here, the impregnation is carried out in an acid electrolyte containing cations of the active material and reducible ions, the redox potential of which is more positive than that of the cations of the active material. In the electrolysis process, the nickel plaque is made the cathode and cations as well as reducible ions (for example nitrate ions) migrate into the pores of the plaque. However, only the reducible ions are reduced because of their more positive potential and during their reduction, hydrogen ions are consumed. This results in precipitation of the cations in the form of active material. This method is a further improvement on previous methods and is adaptable to mass production.
Loading levels could be increased somewhat by repeated electrolytic impregnation and overnight drying between each impregnation. However, this process modification increases manufacturing time. More rapid impregnation could be achieved by increasing the temperature of the electrolyte as described in R. L. Beauchamp U.S. Pat. No. 3,573,101, issued Mar. 30, 1971 and U.S. Pat. No. 3,653,967, issued Apr. 4, 1972. However, attempts to further reduce time of impregnation at such increased temperature by increasing current density results in deposition on the surface of the plaque preventing further impregnation in interior pores. Attempts to achieve higher loadings, as for example, by continued electrolytic impregnation, led either to accumulation on the outside surface of the plaque or to reduction in the percent utility of active material or both. Active material accumulating on the outside surface of the plaque separates during battery operation and does not contribute to the capacity of the electrode.
One of the things that limits the loading and percent utilization of the loading is corrosion caused by exposure of the nickel plaque to the acidic electrolyte. This is particularly serious in commercial manufacturing processes because of the necessity in automatic electrolytic loading apparatus to expose the nickel plaque to the acidic electrolyte for a considerable amount of time. High acidity of the electrolyte is required to prevent premature precipitation of the active cations before they migrate into the pores of the plaque. However, corrosion produces nickel ions, which not only compete for space in the pores with the active cations, but also compete in the electrolytic battery operation. Also, the problem is more serious for the negative electrode on an alkaline battery since nickel ions actively contribute to the capacity of the positive electrode of an alkaline battery but subtract from the negative electrode capacity. Corrosion has other undesirable effects on alkaline battery electrodes, as for example, the structural weakening of the plaque itself and the deposit of large crystals in the pores of the nickel plaque which prevents the usual increase in capacity on recycling.